Resistance measuring devices are known in principle. They are used to measure the internal resistance of energy storage, such as the vehicle battery in a vehicle electrical system of a motor vehicle. A functional check of such energy storage in a motor vehicle is customarily checked by the internal resistance of the energy storage. Checking the internal resistance of the energy storage may cause a malfunction, particularly due to damaged cables or the like, to determine if the internal resistance of the respective energy storage falls below the permissible limits, on or off. Especially in a start/stop system in motorized vehicles and in the required personal electrical system stabilization, the measurement of the internal resistance of energy storage is particularly important. Leaving the internal resistance of an energy storage below a prescribed limit may cause a shutdown of the start/stop system. The energy storage devices to be tested may be fixed or may also be replaceable energy storage. The necessary internal resistance measurement is for voltage sources, such as super-capacitors CAP or conventional automotive batteries.
Particularly for the latest generation of batteries and supercapacitors, the internal resistance measurement is problematic because these energy storage devices are designed for very high power density and are set for the lowest possible internal resistance. With an energy storage device which has a very low internal resistance, however, a large current flow is necessary to produce an appreciable voltage drop. To generate a voltage drop of 0.5 volts (V) to a voltage source with an internal resistance of 1 mega-ohm (MΩ), for example, a flow of current of 500 amperes (A) is required. The low-impedance design of energy storage devices accordingly compels the resistance measurement in the MΩ range requiring costly and high-precision measuring instruments.
A further problem is that the measurement of the internal resistance of the energy storage device during the operation of a connected energy consuming device needs to be accurate. A particularly steep current ramp is needed to enable a reliable internal resistance measurement. In order that the discharge of the energy storage device is not measured, the current pulse must be turned on as “hard” and also be turned off “soft” to avoid voltage spikes (load dump) by a sudden current break.